Reducing Moisture Risk in Retrofit

Breathable IWI solution radical retrofit

A 19th century Yorkshire farmhouse has undergone a radical transformation with a deep ‘whole house’ retrofit undertaken by Green Building Store. The project, located in Cumberworth West Yorkshire, has trialled the use of an innovative internal wall insulation, TecTem, from Knauf Aquapanel in Germany.


The radical retrofit was designed using the Passivhaus methodology, modelled in Passivhaus Planning Package software, and has resulted in a reduction in space heating need of up to 86%. Planning requirements meant that internal wall insulation was needed and modelling of the building showed that there would be risk of moisture build up and ‘interstitial condensation’ unless care and attention was given to the IWI insulation chosen for the project.


The Green Building Store team were able to trial TecTem for the first time in the UK. TecTem is a board material manufactured from perlite, an expanded volcanic mineral, already commonly used in plasters. It was deemed suitable for use on the barn walls exposed to ‘moisture stress’ as it is vapour and capillary open as well as being alkaline, so inhibiting mould growth. The design team found that the risk factor of mould and interstitial condensation was different for different construction types within the project. As a result, the insulation strategies at the project were tailored to the different construction types in the building


Researchers from the AECB have set up HygroTrac sensors in the walls of the retrofit to monitor how well the IWI material handles moisture in the walls and the interim findings to date are that the TecTem is working as expected. In addition, temperature monitoring at the project is showing that ‘the building fabric maintains very constant indoor temperatures, regardless of outdoor climatic conditions’. CO2 monitoring data, used as an indication of indoor air quality, is ‘showing considerably lower CO2 in the retrofit when set against a conventional 20 year old cavity wall property that is naturally ventilated’. The Cumberworth radical retrofit is also the first project to beta test the AECB CarbonLite Retrofit certification system.


Bill Butcher, Project Leader on the retrofit commented: “In many ways, thinking about the best ways to install internal wall insulation is at the current cutting edge of sustainable building in the UK. The UK Government and construction industry has not really fully woken up to the complexity and risk relating to the radical ‘deep’ retrofit. The Cumberworth project offers a great testbed for different IWI approaches.”


Building professionals can download a FREE 25 page technical briefing on the Cumberworth radical retrofit project:



Similar Posts